1. Field of the Invention
The present invention relates generally to electrolytic capacitors, and more specifically to an electric double layer-capacitor of low ESR (equivalent series resistance) and a method for fabricating the capacitor.
2. Description of the Related Art
Attention has recently been directed to the attractive features of electric double layer capacitors. Effort has been made in developing smaller and yet high capacitance types for combination with lead storage batteries for automobile cell motor applications or combination with solar batteries for backup power applications.
An electric double layer capacitor as disclosed in Japanese Utility Model Publication 61 (1986)-117238 consists of a plurality of capacitor cells stacked in a layered structure. As shown in FIG. 1A, each of the prior art capacitor cells is comprised of a pair of polarized paste electrodes 11a and 11b respectively secured to collector electrodes 12a and 12b, forming upper and lower parts of the capacitor cell with their outer edges. Polarized paste electrodes 11a, 11b have their edges offset from the periphery of the collector electrodes 12a, 12b so that stepped portions are formed at the peripheral edges of the collector electrodes. For separating the polarized electrodes 11a, 11b from each other, a separator 10 of porous material is provided as an ion-permeable member, which is slightly larger than the polarized electrodes 11a, 11b. To provide a sealing contact between the upper and lower parts of the cell in order to prevent the leakage of electrolyte, the prior art method involves attaching gaskets 13a and 13b respectively to the stepped peripheral edges so that the separator 10 is disposed between the inner edges of gaskets 13a, 13b and applying pressure to the cell structure in directions towards each other at an elevated temperature. Under heat and pressure, the gaskets 13a and 13b are softened and thermally joined with each other as illustrated in FIG. 1B. In this process, the gaskets are deformed, bulging outwards as illustrated. In order to achieve the necessary sealing, the gaskets must be sufficiently joined together and cooled into a hardened piece. However, if it is desired to achieve the goal at relatively low pressures, the stepped portions of the cell must be of substantial size to allow the gaskets to sufficiently bulge outwards and each gasket must be sufficiently larger in thickness than the polarized electrode. As a result, the separation between the separator 10 and each polarized electrode tends to increase, with an attendant increase in the equivalent series resistance (ESR) of the cell. This could be avoided by using thick gaskets and applying higher pressure. However, the electrolyte inside the cell tends to leak through the gaskets before they are thermally joined together. This results in an increase in the percent defective. Therefore, the ESR and the percent defective contradict each other. The prior art electric double layer capacitor cannot simultaneously satisfy these important factors.
It is therefore an object of the present invention to provide a method of fabricating electric double layer capacitors that can simultaneously meet the requirements of low ESR value and low percent defective.
According to a first aspect of the present invention, there is provided a method of fabricating a component cell of an electric double layer capacitor, comprising the steps of forming a first half portion of the cell by pasting a first polarized electrode impregnated with electrolyte to a first collector electrode so that a first stepped portion is formed on the periphery, of the first collector electrode and attaching a first gasket to the first stepped portion, forming a second half portion of the cell by pasting a second polarized electrode impregnated with electrolyte to a second collector electrode so that a second stepped portion is formed on the periphery of the second collector electrode and attaching a second gasket to the second stepped portion, attaching a third gasket to the periphery of a separator and placing the separator between the first and second half portions of the cell, and applying pressure to the first and second half portions of the cell in directions towards each other at an elevated temperature so that the first and second gaskets are thermally joined with the third gasket.
According to a second aspect, the present invention provides a capacitor cell comprising first and second collector electrode, first and second polarized electrodes impregnated with electrolyte and pasted to the first and second collector electrodes, respectively, so that first and second stepped portions are respectively formed on the periphery of the first and second collector electrodes, a separator disposed between the first and second polarized electrodes, and first and second gaskets attached to the first and second stepped portions, respectively, and a third gasket attached to the periphery of the separator, the first and second gaskets being thermally joined with the third gasket.
According to a third aspect, the present invention provides an electric double layer capacitor comprising a plurality of capacitor component cells stacked in a layered structure. Each of the capacitor component cells comprises first and second collector electrodes, first and second polarized electrodes impregnated with electrolyte and pasted to the first and second collector electrodes, respectively, so that first and second stepped portions are respectively formed on the periphery of the first and second collector electrodes; a separator disposed between the first and second polarized electrodes; and first and second gaskets attached to the first and second stepped portions, respectively, and a third gasket attached to the periphery of said separator, the first and second gaskets being thermally joined with the third gasket.